1. Field of the Invention
The present invention relates generally to the molding of optical glass lenses and, more particularly, to the production of molten optical glass gob preforms to be used in a precision glass molding process.
2. Brief Description of the Prior Art
Various methods and apparatus for the compression molding of glass optical elements are known in the prior art. With these methods and apparatus, optical element preforms, sometimes referred to as gobs, are compression molded at high temperatures to form glass lens elements. The basic process and apparatus for molding glass optical elements is taught in a series of patents assigned to Eastman Kodak Company. Such patents are U.S. Pat. No. 3,833,347 to Angle et al, U.S. Pat. No. 4,139,677 to Blair et al, and U.S. Pat. No. 4,168,961 to Blair. In the practice of the process described in such patents, a glass preform or gob is inserted into a mold cavity. The molds reside within a chamber in which is maintained a non-oxidizing atmosphere during the molding process. The preform is then heat softened by increasing the temperature of the mold to thereby bring the preform up to about 100.degree. C. above the glass transition temperature (T.sub.g) for the particular type of glass from which the preform has been made. Pressure is then applied by the mold to force the preform to conform to the shape of the mold. The mold and preform are then allowed to cool below the transition temperature of the glass. The pressure from the mold is then relieved. The temperature is lowered further and the finished molded lens is removed.
Because precision glass molding of optical elements is done by compression rather than by injection (as is utilized in plastic molding), a precursor metered amount of glass, typically referred to as a preform or gob is required. Although good quality gobs or preforms can be produced through grinding and polishing, it is preferable that a molten gob dropping process be used for economic reasons. U.S. Pat. No. 3,293,017 and U.S. Pat. No. 3,271,126 describe an apparatus and method for fabricating small glass wafers by means of dripping molten glass from an orifice in single drops or gobs into a catching, pressing and cooling mechanism. However, these glass wafers were not intended for optical purposes. Their stated purpose was for ceramic barrier layer capacitors. These patents failed to teach anything specific with regard to the geometry of the nozzle through which the molten glass gobs or preforms are formed with the exception of the drawings which merely depict a straight tubular structure.
In the production of gobs or preforms to be used in a precision glass molding process for molding optical glass elements, it is critical that each gob or preform have good internal quality and that the gobs be repeatable in size. Nothing in the prior art teaches that nozzle geometry can be used to control gob size and improve internal quality of the gob formed therewith.